1. Field of the Invention
This invention relates to the field of electrical direct current distribution systems. More specifically, this invention comprises a method for locating phase to ground faults in DC distribution systems.
2. Description of the Related Art
Ungrounded or high-resistance grounded power systems have many advantages, including continuous operation after a single line to ground fault. While theoretically such an ungrounded DC system can be operated with one phase grounded through a fault, a secondary phase to ground fault on the other phase may occur causing a disruptive fault between the two phases. To reduce the probability of a second ground fault on the other phase, which results in the phase-phase fault condition with high current magnitudes and a consequential sag in supply voltage, it is of great importance to derive methods for fast and robust ground fault location in high-resistance grounded system.
The DC Zonal Electric Distribution System (“DCZEDS”) discussed hereafter is part of a notional all-electric ship integrated power system (IPS) for a destroyer class ship, which includes a 80 MW power generation system, a ship hydrodynamic propulsion system, and models of various ship service loads powered through the DCZEDS.
FIG. 1 shows a simplified schematic representation of a DCZEDS (10). Bus 1 (reference numeral (12)) includes a positive and negative line (or “rail”), as does bus 2 (14). The system is divided into zones. Zone 2 (16) and zone 3 (20) are shown. Zone 2 (16) includes a pair of DC/DC converters (26) which feed power from the two buses to the loads within zone 2, which are labeled collectively as zone 2 load (18). Zone 3 likewise has a group of loads denoted as zone 3 load (22). The loads in zone 3 are also fed by a pair of DC/DC converters (26). High-resistance grounding of the buses in the DCZEDS is implemented to permit continuity of service during a ground fault (The high-resistance grounding is not shown in FIG. 1).
A method for detecting and locating a ground fault in such a system should be fast. However, the method must also be tolerant of the different possible operation modes of the DCZEDS, different system parameters causing unbalances, effects from the inherent noise due to switched converter systems, EMI noise, and other random variables (temperature, humidity, etc.). In addition, faults can be continuous or intermittent with either a constant or a random behavior of the fault resistance (which includes the arc resistance, the contact resistance, and the path resistance) as the characteristic features. The fault resistance in low or medium voltage distribution systems may vary in a wide range from a few milliohms (bolted faults) to several tens of kilo ohms (high impedance faults). It is important to remark that in high resistance grounded systems the fault current is practically independent of the fault resistance.
Previously, the ground fault locating method for an ungrounded system injects an AC signal into the faulted network and traces the signal using hand-held detectors or permanently mounted-in sensors to locate the fault. This method can be introduced into DCZEDS for locating ground faults as illustrated in FIG. 2. AC signal generator (30) is applied to center tap (32) in rectifier (24) (which feeds bus 2 (14)).
The onset of a phase-to-ground fault is detected by monitoring the line-to-ground voltages, since the fault creates a distinct voltage unbalance with respect to ground. After detecting the fault, the AC signal generator is applied for a short time and the resulting AC current is traced throughout the system in order to locate the fault. AC current measurement devices are needed in this method. However, the application of handheld AC current sensors is time consuming and subject to human error. Moreover, using a large number of fixed mounted sensors, necessary for an automated approach, requires sophisticated data communication links and also may reduce the overall reliability. Furthermore, applying this pilot signal approach of fault location to DCZEDS is expected to yield additional problems with locating common rail faults in systems with DC/DC converters which create one common bus and one “switched” bus.
Another possible method may utilize the system impedance over a wide frequency band. A signal source is momentarily applied to the DCZEDS between ground and a central point, such as a rectifier mid-point. The source can either be an adjustable, single frequency AC source or a pulse source. For the frequency domain analysis, the system impedance for a wide range of frequency is monitored by applying Fast Fourier Transforms (“FFT's”) to the voltages and the resulting currents. However obtaining the accurate system impedance from measurements in a short time during system operation is a challenge. The high frequency noise caused by the interaction between Power Electronic (“PE”) switching converters and cables may prohibit a good reading of the system impedance signal which is supposed to provide information regarding the fault location. An extra signal generator is needed for this approach.
Accordingly, it would be desirable to provide a new method for fault location in ungrounded or high-resistance grounded DC distribution systems which neither requires an additional signal source nor requires sensors throughout the system but can still locate the fault quickly and accurately.